The present invention relates to a mobile gravel sorter, which is arranged to move in a direction of travel along a road, comprising a gathering unit which is arranged to gather up granular material from a roadway as the gravel sorter moves in the direction of travel, a sorting unit for sorting and supplying to the roadway the amount of the material that is smaller than a given grain size, which sorting unit comprises a substantially circular drum which is arranged after the gathering unit in the direction of travel and which has a centre axis, an inlet means in connection with the gathering unit and an outlet means which is arranged in connection with the collecting unit and separated from the inlet means in the longitudinal direction of the drum, a collecting unit for collecting material exceeding said given grain size, and a screen cloth means which is arranged to cover openings in the circumferential surface of the drum.
As described in the brochure xe2x80x9croad maintenance bare groundxe2x80x9d issued by the National Swedish Road Administration, the wearing course of a gravel road is worn and ground down under the action of traffic and grading. The coarse material is crushed to a sandy material. The fine material dusts away and some of the gravel material is thrown out on the embankment. The wearing course is transformed into gravel which is sensitive to corrugation and has an excess sand fraction. After some time, the gravel road has such poor standards as concerns the composition and thickness of the wearing course and a reduced runoff of surface water that it is necessary to take measures to improve the wearing course and the water runoff.
A well-balanced cycle of measures to maintain acceptable runoff of surface water and a correct composition of the wearing course is important to obtain the lowest possible total cost of gravel road maintenance.
Today, there are about 284,000 km private roads in Sweden which are covered with a new layer of gravel year after year. This results in high costs and has a considerable impact on the environment, since gravel is getting scarce. The gravel that has been spread out on the roads has not disappeared, but most of it has been pressed out into the ditches.
According to prior-art methods and by using prior-art road machines, the maintenance is carried out by adding gravel and stone material having a fraction of 4-18 mm, which is the ideal size of the gravel and stone material in order to obtain a satisfactory bearing capacity, and possibly by cutting the edges of the roadway and drawing up the thrown-out material. The material that has slid down the embankment has a relatively high share of material with a size in the upper range of said fraction and therefore it is of great interest to recover this material.
The drawn-up material sometimes comprises a high amount of turfs and contains relatively large stones, and herefore it cannot be used directly since such a composition of material on the road would result in a road with too poor a bearing capacity.
In the brochure xe2x80x9croad maintenance bare groundxe2x80x9d, two different ways of treating the drawn-up material are described.
According to the first alternative, the line of drawn-up material is loaded into the vibrating grate bucket of a wheel loader, by means of which too large stones and turfs are sorted out. After sorting, the remaining material is emptied onto the surrounding ground, where possible. This method requires a road grader, which cuts the edges of the road and draws back the material, and a wheel loader with a vibrating grate bucket, which means that two drivers are needed.
According to the second alternative, use is made of a so-called stone picker (schematic view, see FIG. 6). The stone picker is attached directly to the road grader or pulled by a separate tractor running after the road grader. Such a stone picker, which is designed to pick up stones in a field, has a number of arms rotating about a shaft which is arranged parallel to the surface of the ground and transversely of the longitudinal direction of the road. The arms encounter stones and turfs in the line of drawn-up material and throw them into a container. When the turfs are thrown into the container, a great amount of gravel is entrained.
According to the first alternative, a succession of machines and thus a number of drivers are required. According to the second alternative, too large quantities of gravel disappear.
SE-451,207 discloses a sorting machine for immediately reusing road gravel in material originating from road maintenance, such as material from graded road sides and edges. The sorting machine has a collecting assembly which collects the material, a conveyor belt which conveys the material from the collecting assembly up to a tumbler which is horizontally arranged and which separates the road gravel and puts it down on the roadway. The sorting unit further has a second conveyor belt which conveys undesired stones and the like from the collected material up to a container. The sorting unit is provided with a motor and is self-propelled. Also this construction requires two operators, one driving the road grader and another driving the sorting unit.
Both the stone picker and the sorting unit are insufficient in case irregular quantities of material have been graded off along the road. They are not capable of levelling the quantities along the road, and the separated roadway gravel will also be spread out irregularly.
One object of the present invention is to provide a solution of the above-mentioned problems.
Another object is to provide a solution which requires as few persons (drivers) as possible and which minimises the need to add new gravel.
These objects are achieved by means of a mobile gravel sorter, which is of the kind stated by way of introduction and which is characterised in that the sorting unit comprises a substantially circular drum which is arranged after the gathering unit in the direction of travel and which has a centre axis, an inlet means in connection with the gathering unit and an outlet means which is arranged in connection with the collecting unit and separated from the inlet means in the longitudinal direction of the drum, at least one screw conveyor which extends in the drum between the inlet means and the outlet means about a helical axis which is substantially concentric with the centre axis of the drum, and a screen cloth means which is arranged to cover openings in the circumferential surface of the drum.
Preferred embodiments of the invention are stated in the dependent claims.
In the radial direction, the screw conveyor advantageously has an extension that is smaller than the inner radius of the drum and extends from the inside of the circumferential surface of the drum so that an axially directed return chamber forms about the centre axis between the inlet and the outlet means of the drum. As a result, it will be possible, when a great amount of material is fed into the gravel sorter, for the amount of the material that exceeds the volume of the defined space to be returned to preceding helical turns, and therefore there is time for all the material to be processed and passed through the meshes of the screen cloth.
Preferably, the drum and the screw conveyor are arranged to rotate together as one unit. Consequently, the problem of material getting stuck between two elements that are movable in relation to each other is avoided, which prevents the sorting unit of the gravel sorter from jamming.